guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Conoscopy
4.2.5.2 Conoscopy of optically anisotropic crystals
In the conoscopic mode, waves deviating from vertical incidence by increasing tilt angles
travel increasing distances within the birefringent crystal. According to the relationship Γ = d
* (n z ' – n x ') described in chapter 4.2.3, it has to be concluded that the retardation Γ of the
waves increases with increasing angles due to the continuous increase of d' (= length of the
light path in the crystal plate). Accordingly, the interference colours in a conoscopic
interference figure should generally increase outwards. However, the interference figure is to
a much larger extent controlled by the orientation of the vibration directions and the
birefringence values of the wave couplets within the observed volume of the anisotropic
crystal. As described previously, the birefringence for waves parallel to optic axes is zero.
It increases as the angle between the optic axis and ray propagation direction (or wave
normal, to be precise) increases.
Raith, Raase & Reinhardt – February 2012
Figure 4-48: Generation of an interference figure in the upper focal plane of the objective by
imaging of parallel sets of rays that pass the crystal plate at different angles. The example
shows these relationships for a uniaxial crystal (calcite) cut perpendicular to the optic axis.
The geometry of interference figures obtained from anisotropic crystals can be illustrated
with the model of the skiodrome sphere developed by Becke (1905). The crystal is considered
to occupy the centre of a sphere. Each ray propagation direction of light waves within the
crystal has a corresponding point on the spherical surface where the ray pierces that surface.
In each of these points, the vibration directions of the related waves can be shown as a
tangent (e.g., O and E in case of optically uniaxial minerals). When connecting all the
tangents of equal vibration direction, a geometric mesh of vibration directions is generated
that depends on the optical symmetry of the crystal (Fig. 4-49,50).
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